The present invention generally relates to vacuum pumps, and more particularly, to pumps known as turbomolecular pumps characterized by xe2x80x9cbladedxe2x80x9d rotor and stator construction with running clearances in the millimeter range which are particularly effective in the free molecular flow range. More specifically, this invention is directed to a novel rotor design and bearing system useful in vacuum pumps of the single-ended type in which the rotor is supported by bearings at either end in a so-called xe2x80x9cstraddle-mountxe2x80x9d arrangement.
High rotational velocity vacuum pumps are used today in many high vacuum applications, where a high vacuum of best quality is required, as well as when a relatively large amount of gas is intended to be pumped away. Typically turbomolecular pumps are used in combination with a higher pressure xe2x80x9croughing pumpxe2x80x9d in a series arrangement, where the turbomolecular pump is connected to the volume to be evacuated at one of its ends and at its other end to the roughing pump. Turbomolecular pumps utilizing a rotor with multilple rows of blades operate according to the principle that fast moving solid surfaces transfer a linear momentum to substances in gas phase which come in their path. Turbomolecular pumps can in this manner impart a velocity to gas molecules and thereby direct gas molecules out from a volume to be evacuated.
A drag stage is commonly incorporated into modern turbomolecular pumps downstream of the bladed rotor portion. Drag stages normally operate at higher pressures by means of friction against a rotating wall forcing gas particles into an arrangement of helical grooves in an adjacent static member. Different types of pumps or stages can also be combined in several steps to give a higher compression. Common for all of these types is that they demand extremely fast rotating parts.
In most turbo molecular pumps, the pressure ratio varies exponentially with the rotational speed of the rotating parts, and the pumping speed varies linearly. Therefore it is highly advantageous to be able to operate at as high of rotational speed as possible. However, higher rotational speeds translate into higher rotor unbalance loads which must be carried by the bearings, and thus the bearings become critical from an operational design standpoint.
Manufacturers have traditionally used mechanical ball bearings and the like to support the rotor because of their reliability and relatively low cost. However, mechanical bearings have certain inherent deficiencies which limit their use in high speed turbomolecular pumps. For example, ball bearings inherently consume power as a result of friction induced by the balls orbiting around the axis at very high angular velocity, causing substantial contact forces between the balls and raceway. Also, power consumption can increase dramatically when increased radial or axial loads are imparted to the bearings from rotor unbalance or from operation at harmonic modes of the system. The internal contact forces in ball bearings also inherently cause wear of the bearing parts and heat generation, both of which are exacerbated by rotor unbalance. Therefore precise balancing of the rotor and alignment of the bearings becomes paramount when using ball bearings.
High vacuum pump manufacturers have progressed from solely using mechanical bearings, to using different types of magnetic bearings, sometimes in combination with mechanical bearings. Generally, so called active magnet bearings, i.e. electronically regulated electro-magnet bearings, are used. Occasionally, so called passive bearings, where the force usually is generated by repulsion between appropriately magnetized permanent magnets, are used. In designs where a pair of magnetic bearings is used to support the rotor, an additional axial stabilizing feature is required, such as a ball bearing or a pair of opposed jewler""s bearings, or alternatively a regulated axial electro-magnet. A disadvantage of this type of design is that the rotor is inherently unstable despite the use of eddy current damping, usually necessitating additional external damping means to prevent large rotor excursions and blade rubbing. Alternatively the rotor may be supported radially by one permanent magnet bearing and one mechanical bearing such as a ball bearing, in which case the mechanical bearing may also provide stability to the rotor. In such designs, additional external damping devices may not be necessary. The advantage with passive systems in general is the lack of complex regulating systems associated with the electromagnet bearings and a lower price.
Turbomolecular pumps have seen wide use in both the semiconductor and medical industries. Generally speaking for these applications the size and weight of the vacuum pumping equipment has not been a significant design factor. The smallest and lightest turbomolecular pumps currently available are in the range of 5 pounds in weight, and in a size range of 4 inches in diameter by 6 inches in length. Also, power consumption has generally not been a driving factor determining the design of prior art pumps.
However, for certain new applications, such state of the art turbomolecular pumps are not satisfactory. For example, turbomolecular pumps are desired for certain space-based applications in which greatly reduced weight, size, power draw, and complexity are all critical. A suitable turbomolecular pump for such applications preferably would weigh less than xc2xd pound, and draw less than 7 Watts power while pumping a chamber to typical high vacuum levels. Regulated electro-magnet type designs are unsuitable because of problems associated with scaling down the internal regulating systems, and the overall complexity of such systems. Existing systems that use mechanical bearings, or mechanical bearings in combination with passive magnetic bearings, fail because of the high relative power consumption of the mechanical bearing, or because of the need for additional external dampers with a two passive bearing system.
Thus, a need exists for a simple, efficient, compact, and lightweight turbomolecular pump.
In one embodiment of the invention, a turbomolecular pump is provided comprising a housing having an intake for connecting to a chamber to be evacuated, and an exhaust for exhausting to lower vacuum. A rotor is disposed within the housing having a high vacuum end exposed to the intake, a low vacuum end, and a plurality of rows of rotor blades disposed about an outer periphery thereof. A non-contacting type main bearing is provided for rotatably supporting the rotor substantially at the high vacuum end; and a contacting type bearing rotatably supports the rotor substantially at the low vacuum end. Preferably the non-contacting bearing is a passive magnetic bearing, and the contacting type bearing is a ball bearing. The pump may be configured wherein the non-contacting bearing is substantially axially nearer the rotor""s axial center of gravity than the contacting bearing. The noncontacting main bearing is exposed to the high vacuum of the intake. An emergency main bearing is provided for supporting the inlet end of the rotor in the event of large rotor excursions.
In accordance with the present invention a serpentine gas flowpath is defined. An annular pumping section defining a first gas flowpath axially extends in a forward-to-aft direction from an inlet end in fluid communication with volume being evacuated, to a pumping section outlet end. An annular drag stage defines a second gas flowpath axially extending in an aft-to-forward direction from an inlet end in fluid communication with the pumping section outlet end, to a drag stage outlet end. A third gas flow path extends axially in a forward-to-aft direction from an inlet in fluid communication with the drag stage outlet end, to an exhaust port.
In another embodiment of the invention, a method of machining the rotor blades of a high speed vacuum pump rotor is provided, comprising the following steps: An axisymmetric rotor blank having an outer surface is provided. A plurality of closely spaced helical grooves are machined in the outer surface of the rotor blank extending along a desired pitch angle, the walls of the helical grooves defining the working surfaces of the rotor blades. A plurality of adjacent annular grooves are machined in the outer surface, intersecting the helical grooves, whereby the annular grooves separate the rotor blades into distinct rows and define the leading and trailing edges of the rotor blades. Preferably the steps of machining the helical grooves and machining the annular grooves are performed in a numerical control lathe operation, using a single point cutting tool for each step.
In still another embodiment of the invention a method is provided for machining the rotor blades of a high speed vacuum pump rotor, comprising the following steps: An axisymmetric rotor blank is provided and a plurality of adjacent annular grooves are machined in the outer surface. A plurality of angled regularly spaced slits are cut into the rotor blank material between annular grooves, the slits defining the working surfaces of the rotor blades, and the annular grooves separating the rotor blades into distinct rows. The slits may be formed by plunging a high speed rotary saw blade radially into the rotor blank to the depth of a rotor blade.